feat(NO-TASK): Announce a new release at shell startup

[aaronware]

The existing notifier only speaks after someone runs a command, so the person most likely to be out of date — the one who has not opened the CLI in a fortnight — is exactly the one it never reaches. This emits a snippet for the shell profile via linchpin shell-init, so a release is announced by opening a terminal instead.

Why it is safe in a profile

• No Node on the startup path. The common case is test plus cat against a pre-rendered file. Starting the CLI to decide whether to say anything would put ~100ms in front of every new prompt, several times a day, to print nothing on all but a handful of them.
• Nothing in the foreground. The refresh is detached and redirected to /dev/null, so a slow or unreachable registry cannot hold up a prompt.
• Nothing is terminal-specific — Ghostty, Terminal.app, iTerm, VS Code and Herd's terminal all just start the user's shell.

syncNoticeFile keeps the notice in step after every command and skips a write that would not change the file, so the steady state is one small read. A copy that cannot update itself — a source checkout, an npx run — neither writes nor clears it: the notice was written by the global install, and a linked working tree wiping it would silence a release for a shell that had nothing to do with that checkout.

Provenance

This was in-flight work already in the working tree, separated out and committed as its own branch so it stays clear of the security review in #69. The code and its tests are unchanged from that working state — I authored the commit message from the code, so check it describes the intent correctly.

Verification

121/121 tests pass, typecheck clean, on this branch alone.

🤖 Generated with Claude Code